Modern optical communications systems typically use many separate but interconnected optical transmission lines. These optical transmission lines are typically interconnected using branching units which allow selected optical signals to be removed from any one optical transmission line and inserted in to another connected optical transmission line.
Current optical branching units typically employ optical components which permit selected portions of optical signals to be removed from a transmission line and inserted in to another transmission line on the basis of wavelength. This form of branching unit is particularly suited to use with wavelength division multiplex (WDM) signals comprising a plurality of distinct but closely spaced wavelength channels. The branching unit may be configured to drop and add specific WDM wavelength channels from a particular transmission line.
FIG. 1 illustrates an example of such an arrangement. The branching unit generally denoted 1 comprises a first circulator 2 a second circulator 3 and an intermediate reflective grating 4 placed in between the two circulators. The first circulator 2 has one optical input 5 and two optical outputs 6 and 7, while the second circulator 3 has two optical inputs 8 and 9, and one optical output 10. One optical output 7 of the first circulator 2 is in optical connection with one optical input 8 of the second circulator 3.
In use, when a WDM optical signal 11 is input to optical input 5 of the branching unit, the entire signal is output at optical output 7 of the first circulator 2. Selected wavelength channels, such as channel 12, are subsequently reflected by the reflective grating 4 and redirected in to the circulator 2 for output at optical output 6. Thus one selected wavelength channel may be dropped from the input signal 11 by appropriately choosing the wavelength reflectivity of the reflective grating 4.
The remaining components 13 of the optical signal 11 then pass to an optical input 8 of the second circulator 3, and are combined with an optical channel 12 which is input at the other optical input 9 of the second circulator 3. The channel 12 is reflected by the reflective grating 4, in a manner similar to the reflection of the corresponding channel 12 output from the optical output 7 of the first circulator 2. The combined optical signal 15 is then output at the optical output 10 of the second circulator 3. Thus, a single optical channel 12 may be dropped from, and added to, optical signal 11.
Additional optical channels may be dropped from optical input signal 11 by employing additional reflective grating elements placed in line with grating element 4 which each have a pre-selected reflectivity appropriate to the channel to be reflected thereby. Clearly, a disadvantage of such an arrangement lies in the fact that in order to be reflected by a given one reflective grating, each pre-selected channel must traverse at least one such grating two times.
The first traversal occurring as the optical channel approaches the grating, the second traversal occurring after reflection thereby. Each such traversal incurs loss in the respective channel, and therefore the greater number of traversals required, the greater the loss incurred. If a larger number of wavelength channels are required to be dropped, a correspondingly large number of gratings will be required. In such circumstances the losses may be high and often produce a large “loss tilt” whereby the channel reflected in the last grating suffers a much higher loss than the channel reflected in the first grating. Channels reflected by intermediate gratings suffer a loss intermediate these two extremes and the result is a strong drop-off, or “tilt”, in the intensities of successive channels.
It will be noted that in the branching unit illustrated in FIG. 1, the wavelength channel 12 that is dropped from the incoming signal 11 is the same wavelength channel that is subsequently added by the second circulator of the branching unit to provide output signal 15. This is necessary in order to avoid optical hole burning in the optical signal output by the branching unit as would occur where the hole 16 in the intermediate signal 13 of the branching unit not filled by the optical channel 12 added thereto by the second circulator 3 of the branching unit. Clearly, this seriously reduces the network routing capacity of any network employing branching units of this type since any channels removed from an optical transmission line by the branching unit must be subsequently added to the transmission line by the same branching unit.
Additionally, wavelength dependant add and drop branching units based on reflective gratings are generally difficult to manufacture, and are limited in the number of channels they may add and drop. Indeed, due to problems associated with chromatic dispersion in the reflective grating implied in such branching units, data transmission rates are limited to 2.5 gigabits per second through such branching units.